1. Field of the Invention
This invention relates to a novel low-brightness product comprising a low-structure magnesium silicate and a silica leach residue or SLR, optionally containing soluble iron, which is especially useful as a filler for newsprint. This invention also relates to a method for its preparation from a mineral source containing soluble magnesium, e.g., the mineral serpentine or other minerals, and an alkali metal silicate, and a method for using the novel filler in low brightness of paper.
2. Description of the Background
Low-cost, low-brightness fillers for use in ground wood specialties and newsprint are known. The paper industry is showing an increasing need for a lower cost, lower brightness filler of this type which will provide comparable performance with higher cost materials prepared from silica, such as the commercial product Zeolex.RTM., a trademark of the J. M. Huber Corporation. The present invention meets this need by providing a low-brightness product comprising a low-structure magnesium silicate SLR filler prepared from a low cost mineral raw material containing soluble magnesium, and optionally soluble iron, and silica, such as the mineral serpentine.
The precipitation of magnesium silicate from sodium silicate and a magnesium salt is known and has been described in the literature (e.g., U.S. Pat. No. 3,401,015 to Ninger). However, in most cases the processes described in the prior art are tailored for the specific purpose of obtaining pure magnesium silicate as in the case where the product is intended for use as a pharmaceutically-acceptable carrier for a drug. In these products the presence of other components is undesirable. Other processes produce magnesium silicate lacking a low-structure as defined in Wason, J. Soc. Cosmet. Chem. 29:497-521 (August 1978), the entire content of which is incorporated herein by reference, and/or contain a silicate but not SLR, and optionally iron.
Specific compositions of magnesium silicate are also known (Vournazos, Hydraulischer und weisser Magnesiazement, Zeitschrift fur anorganische und allegemeine Chemie 200:237-244(1931); Strese et al, Synthese von Magnesium-silikatgelen mit zwei dimensional regelmassiger Struktur, Zeitschrift fur anorganische und allgemeine Chemie 247:65-95(1941)). Other authors have reported that the compositions of the magnesium silicate product vary depending on the precipitation conditions. Thus, the composition of the final product has been described as a sodium magnesium silicate (Gmelins Handbuch der anorganischen Chemie, System 27, Mg, Teil B, Lieferung 3, Weinheim, Verlag Chemie GmbH, p. 443 (1938)) and magnesium silicate (Vournazos, Hydraulischer und weisser Magnesiazement, Zeitschrift fur anorganische und allegemeine Chemie, 200:237-244(1931); Strese et al, Synthese von Magnesium-silikatgelen mit zwei dimensional regelmassiger Struktur, Zeitschrift fur anorganische und allgemeine Chemie 247:65-95 (1941); Hinz et al, Uber die Fallungsprodukte von Magnesiumsalz- und Wasserglas-losungen, Silikat Technik 8:148-156(1957)).
In later work it was shown that the molar ratio of the produced magnesium silicate is the same as the molar ratio of the sodium silicate used in the precipitation (Roseman et al, Journal of the American Pharmaceutical Association 99:271-277(1940)). The sodium oxide content of the final product is said to be low if the magnesium salt is in excess in the precipitation and increases with a sodium silicate excess (Hinz et al, supra).
Fine-particle, synthetic silicas are amorphous, submicron size, white powders. These products have been referred to interchangeably as silicas, hydrated silicas, silicon dioxide, silicic acid, fillers, silicates, silica pigments, xerogels, aerogels and amorphous silicas, among other names.
A distinguishing factor among silicas is the method utilized for preparing the silicas or silicates. When silicas are precipitated from a solution or slurry their structure and associated properties can be controlled. These controlled structure silicas have been named Very High Structure (VHS), High Structure (HS), Medium Structure (MS), Low Structure (LS) and Very Low Structure (VLS) Silicas. Each group has diverse characteristics which are related to their structure such as particle size, water porosity, BET surface size and silanol group density, among others.
Low-Structure Silicas (LS) or silicates have a characteristic Water Pore Volume (PV), Structure Index (SI: amount of water associated with 100 parts of the dry recoverable product) and % Wet Cake Moisture (% WCM: structural water or the amount associated with a washed silica precipitate) (Wason, J. Soc. Cosmet. Chem., supra). The characteristic values of these parameters distinguish LS from other types of silicates.
Asbestos tailings from serpentine mineral wastes are known as a source of magnesium salts with mineral acids (U.S. Pat. Nos. 2,402,370 to Chalmers, 3,320,029 to Adams and 4,058,587, 4,277,449 to Lalancette and Canadian Patent No. 1,034,385 to Marek et al). Moreover, the methods utilized by the prior art separate the soluble salts from any insoluble residue (e.g., U.S. Pat. No. 4,277,449). Thus, many of the prior art products are not suitable for use as a newsprint filler, and those which would be do not have the composition, low-structure, low-brightness characteristics of the filler of the invention.
Accordingly, there still is a need for a magnesium silicate filler formed by precipitation which has superior characteristics for use as a low brightness filler for newsprint.